Acoustical signal generator



Sept. 12, 1967 J, BREGUET 3,341,842

ACOUSTICAL S IGNAL GENERATOR Filed Aug. 27, 1965 5 Sheets-Sheet 1 Sept. 12, 1967 J. F. BREGUET 3,341,342

ACOUSTICAL SIGNAL GENERATOR Filed Aug. 27, 1965 5 Sheets-Sheet 2 Sept. 12, 1967 5 Sheets-Sheet 5 Filed Aug. 27, 1965 FIGA p 1967 F. BREGUET 3,341,842

ACOUSTICAL S I GNAL GENERATOR Filed Aug. 27, 1965 5 Sheets-Sheet 4 Sept. 12, 1967 J. F. BREGUET 3,341,842

ACOUSTICAL SIGNAL GENERATOR Filed Aug. 27, 1965 5 Sheets-Sheet 5 ss 85 0 64A,

' A 76 88a 87 so United States Fatent O 3,341,842 ACOUSTICAL SIGNAL GENERATOR Jean F. Breguet, Le Locle, Switzerland, assignor to Angelus S.A., Le Locle, witzerland Filed Aug. 27, 1965, Ser. No. 483,130 Claims priority, application France, Aug. 28, 1964, 11,290/64; Oct. 19, 1964, 13,536/64; May 19, 1965, 6,997/65 9 Claims. (Cl. 346-384) ABSTRACT OF THE DISCLOSURE An acoustical signal generator comprising acoustical signal means; voltage supply means; a transistor having a base electrode and two working electrodes comprising a collector and an emitter; a transformer having a primary with at least two terminals and a secondary with at least one secondary winding and at least two terminals; a primary circuit comprising means connecting one terminal of the primary with one terminal of the voltage supply means, means connecting another terminal of the primary with one working electrode of the transistor and means connecting the other working electrode of the transistor with the opposite terminal of the power supply means, and a secondary circuit comprising means including an impedance connecting one terminal of the secondary with one working electrode of the transistor and connecting another terminal of the secondary with the base electrode of the transistor; the acoustical signal means being connected in the secondary circuit for operation by secondary current of the transformer.

The object of the invention is to simplify known acoustical generating circuits which have heretofore had a relatively complicated construction, and to provide a circuit arrangement consisting of only a few components and permitting a versatile use of the signal means, for example as a bell, as a warning signal or as a whistle or horn in toy vehicles, especially electric toy railroads.

For this purpose the acoustical signal system according to the invention is characterized in that the acoustical signal means is in the secondary circuit of a transformer and the base of a transistor is connected to one terminal of a secondary winding of the transformer, while the other terminal of the secondary winding is connected through an impedance member with one working electrode (emitter or collector) of the transistor.

The invention can be designed in two basic circuit variants; either the impedance member is formed by the signal means itself, which is then a loudspeaker of high impedance, or the transformer is providedin addition to the secondary winding connected for feedback to the primary windingwith a second secondary winding to which the acoustical signal means can be connected. In the latter case, a loudspeaker of low impedance is preferably used as the signal means.

The invention may be designed as a self-enclosed, battery-fed independent unit having an external switch contact. This signal unit can then be used for example as a door hell or calling signal in oflices or factories. The selfexcited oscillator is switched either through a hand switch or automatically, for example through a photo-cell, an electrically actuated switch, or through a mechanically movable actuating member which is coupled in suitable manner, for example the handle or the lock of the door on which the signal means is mounted.

As a self-enclosed unit, the signal generator according to the invention may be installed also in toys of all kinds,

for example in dolls or animals, and then permits the production of sounds characteristic of the respective toy.

Similarly, the acoustical signal means and the oscillator furnishing an audio frequency current to it may be arranged at separated locations so that a loudspeaker serving as signal means is operable from a distance. This application is of interest especially in electrical installations for toy vehicles, in particular electric railroads. According to another inventive idea, the feeding of a loudspeaker arranged stationary next to the tracks or on a vehicle, for example the locomotive is effected through at least one conductor of the main circuit of the installation, for example a running rail track, while the other electric connection between the loudspeaker and the transformer secondary circuit of the oscillator occurs either through a separate line or through the other conductor of the main circuit of the electrical installation, for example the third rail. In the first case, a loudspeaker of high impedane is applied directly to the single secondary winding of the transformer, while in the second case the transformer has a second secondary winding from which audio frequency current for the loudspeaker is available. The terminals of this second secondary winding are then connected to the running rail track and the third rail of the electric railroad installation, and audio frequency current for the loudspeaker is superimposed on the main circuit for the traction current. Suitable circuit elements are then provided so that the audio frequency current for feeding the loudspeaker can flow neither into the main feed source nor into the motor circuit of the vehicle, while on the other hand, provision is made so that the traction current cannot influence the function of the loudspeaker.

The invention is explained in greater detail with reference to several embodiments shown by way of example in the drawings, in which:

FIG. 1 is a circuit diagram of a first form of the electric audio frequency oscillator, in which a loudspeaker of high impedance is connected as signal means directly to the single secondary winding of the transformer;

FIG. 2 is a circuit diagram of a second form of an electric audio frequency oscillator, where the audio frequency current for feeding the acoustical signal means is furnished through a second secondary winding of the transformer;

FIG. 3 is a schematic sectional view of a door signal which functions with the acoustical signal generator according to the invention;

FIG. 4 is a circuit diagram of a variant of the oscillator according to FIG. 2 for connection to the rails of an electric toy railroad;

FIG. 5 is an electric in which is mounted a according to FIG. 4;

FIG. 6 is a circuit diagram of the installation of an electric toy railroad where stationary loudspeakers as well as loudspeakers mounted on a vehicle are used, one electric connection of which to the stationary oscillator occurs through a running rail of the tracks; and

FIG. 7 is a circuit diagram of another electric railroad installation where a loudspeaker mounted on a vehicle is fed through a running rail of the track and a third rail.

The circuit shown in FIG. 1 presents an oscillator generally designated by 1, which comprises a transistor 2 and a transformer 3. The primary winding 4 of the transformer 3 is in the output and the secondary winding 5 in the input of the transistor 2. The feedback of the oscillator thus occurs through the transformer 3. p

The transformer 3 is fed through a current source 6 which is connected in series with the primary winding 4 and with the collector-emitter section of the transistor 2.

circuit diagram of a rail vehicle loudspeaker fed by an oscillator In addition, a switch 7 is connected in series with the current source 6. A resistor 8 arranged in the base circuit of the transistor 2 serves to provide a base bias for the transistor. The secondary winding of the transformer is connected through a loudspeaker 9 of high impedance and through ground with one of the working electrodes (emitter, collector) of the transistor 2.

When switch 7 is closed, self-excitation of the oscillator sets in and the loudspeaker 9 is set in operation. To obtain self-excitation, the coils 4 and 5 of transformer 3 are preferably overcoupled, but the transformer may alternatively have an ordinary co-coupling or may have a saturable iron core. In the latter case, the electric circuit grounding the secondary winding 5 includes an ohmic resistance as impedance member (FIG. 4).

In any case, the voltages induced in the secondary winding 5 upon excitation of the oscillator are sufiicient to actuate the loudspeaker 9 directly at high impedance. By tapping the high voltage induced in the secondary winding directly through a loudspeaker of high impedance, a new application of a blocking oscillator arrangement is realizable in a particularly economical manner. Depending on the purpose, the sound of the loudspeaker can be modified in a simple manner by varying the transformer or resistance values.

In the second example according to FIG. 2, the circuit arrangement of FIG. 1 is modified and supplemented in that the loudspeaker of high impedance is replaced by a condenser 10 and transformer 3 has an auxiliary secondary winding 11 at the terminals 12 and 13 of which a useful voltage is available. The auxiliary secondary winding 11 thus furnishes an audio frequency current of low frequency, which again can either be used directly for feeding a loudspeaker of high impedance or can be first fed into another circuit, where it is imposed on other current flowing therein. A loudspeaker connected at any point of this circuit can then be operated as desired. The circuit may in particular be the power circuit of an electric toy railroad, as represented schematically in FIG. 2. Then the terminal 13 is connected to the central third rail 31 of a railroad track, while terminal. 12 of winding 11 is connected through a control unit 32 with the running rail 30 forming the other conductor. A condenser 33 bridging the terminal of the control unit 32 permits the feeding of the audio frequency current into the conductors 30 and 31 even when the main feed current of the installation is cut off. The control unit 32 may be a simple switch, actuated by hand or automatically by the running train.

In FIG. 3, the application of the circuit arrangement of FIG. 1 to a door bell is illustrated. The door bell has a housing 14, which contains the electric oscillator of FIG. 1, as well as a battery feeding the oscillator (not shown in FIG. 3). A plate 15 is fastened to one side of a door 16 and serves as a support for the housing 14. A threaded metal sleeve 17 extends through the door and is secured by a nut 19 on the end projecting into the housing 14. The outer end of the sleeve 17 protruding on the other side of the door has a flange part 20, into which is clamped a perforated ring of insulating material receiving the bell button 22. A tube 21 of insulating material is inserted in the interior of the metal sleeve 17. A metal conducting pin 18 is slidable in the inner end portion of the sleeve 17 opening into the housing 14 and is insulated from the sleeve by the lining 21. The free end of pin 18 protrudes from the sleeve 17 and engages a contact plate 24 which is fastened on the inner wall of the housing 14. There is inserted in the interior of the tube 21 a helical spring 23, one end of which rests on the inner end of pin 18 and the other end of which bears on the inner end of a metal bell button 22, normally pressing the latter outwardly against the insulating ring 25. In this inoperative position, the bell button 22 is lifted off the end face of the metallic tube 17, so that there is no electrically conducting connection between the metal lic tube 17 on the one hand and the contact plate 24, on the other.

The metallic bell button 22 thus forms the switch 7 of FIG. 1, the metallic tube 17 (in the representation of FIG. 1) being grounded, for example while the contact plate 24 is connected to one pole of the battery 6 (according to FIG. 1). When the bell button 22 is pressed in against the action of spring 23, a conductive contact closure is established between the conducting sleeve 17, the bell button 22, the spring 23, the metallic pin 18 and the contact plate 24, so that the oscillator starts to swing and the signal means is energized. The spring 23 here has the double function of a mechanical as well as an electrical connection between the contact plate 24 and the button 22, so that the entire bell system described can be used without any conversion, regardless of the thickness of the door 16.

Apart from this structural feature, the bell has the great advantage that while producing a uniform ring, it absorbs only about one tenth of the current required for known electro-mechanically functioning bell systems. Moreover, the sound of the hell can be adjusted in simple manner to the particular acoustical taste of the user, as has been described above with reference to FIG. 1. Also the installation of the bell according to FIG. 3 is very simple and economical and can readily be adapted to all types of doors. Instead of being operated by hand,

the hell can be operated automatically, for example by a photo-cell or mechanically as the door latch is pressed or the lock opened.

In FIG. 4, an application of the circuit principle according to FIG. 2 to an electric model installation, for example an electric railroad, is represented in greater detail. An oscillator unit 1 is shown as comprising a transistor 42 and a transformer 43, which has a primary coil 44 and two secondary coils 45 and 46. The feedback of the self-excited oscillator occurs through the secondary winding 45 lying in the base circuit of the transistor 42 and connected through a resistor 40 with the emitter of the transistor 42. The primary winding 44 lies in the collector circuit of the transistor. The induced A.C. voltage serving to feed a sound generator is tapped through the second secondary winding 46. In the example illustrated, the sound generator is disposed on an electrically driven toy vehicle, for example a car fed through a third rail or an electric locomotive, the electric wiring of which is shown in FIG. 5. This vehicle runs on the rails 52, comprising rails insulated from one another and connected to the unit 1 through the junction lines 51. A condenser 49 in the circuit of the secondary winding 46 of the transformer prevents passage of the traction current feeding the rails 52 for the electrically driven toy vehicle.

According to FIG. 4, the oscillator 1 is fed through a transformer 53 connectable to the power supply. A diode 47 arranged in the output circuit of the transistor 42 provides for a rectification of the alternating current. A switch 48 permits the oscillator 1 to be switched on by hand. In addition, for the automatic starting of the oscillator 1, there is provided a control relay which, separated both from the oscillator and from the sound generator, is housed in a special switching station and is operable for example by the running train or in connection with a railroad signal. On the other hand, this control relay 55 can be simply remote-controlled.

The transformer 53 serves at the same time to supply the rails 52 with traction current, which is supplied to the rails through unit 1 and the previously mentioned junction lines 51. This traction current is tapped off of the transformer 53 through a sliding contact 31, which serves in known manner to control the traction current strength and hence the vehicle speed. Onto this traction current of cycles which may serve at the same time for feeding additional electrical installations, for example lamps, the audio frequency current from the secondary winding 46 of transformer 43 is superimposed in unit 1. As has been 3 mentioned, the condenser 49 prevents the passage of the 60-cycle current to the transformer 43 of the oscillator and has so small a capacity that the function of the oscillator is practically not influenced. In order, on the other hand, to prevent the audio frequency current from flowing from the oscillator through the traction current feed line into the secondary winding of the feed transformer 53, there is inserted in the traction current connecting line between transformer 53 and oscillator 1, a choke coil 54 which constitutes so high an impedance for the audio frequency current from the oscillator that practically no audio frequency energy can flow through this choke coil into the power supply transformer 53.

In FIG. 5, there is shown the circuit diagram of an electric locomotive running on rails 52 and containing a sound generator 65 which is shown as a loudspeaker. As has already been indicated in FIG. 4, the two conductors of rail 52 are fed through the junctions 51 with the traction current as well as the audio frequency current superimposed on this traction current by the oscillator 1 according to FIG. 4. The supply of current to the electric locomotive occurs on the one hand through the two wheels 62 and 63 and, on the other, through a current collector 64 which slides on the currentcarrying central third rail. In the arrangement according to FIG. 5, the signal generator 65, which is for example a loudspeaker with a low impedance of about 8-15 ohms, the electric motor 66, and an incandescent lamp 69 are connected in parallel. In the loudspeaker circuit, which thus extends between wheel 62 and the current collector 64, a condenser 67 is arranged to prevent the traction current and the illuminating current from interfering with the function of the loudspeaker, while in the motor circuit, which extends between the slip contact 64 and the other wheel 63, a choke coil 68 is connected in series and has a high impedance which prevents the audio frequency current from the loudspeaker circuit from flowing into the motor windings. In some instances, the impedance of the motor windings themselves may be sufficient to prevent passage of the audio frequency current.

In the example according to FIGS. 4 and 5, it has been assumed that the traction current is an alternating current. Naturally it is readily possible also to operate with a direct current as traction current. In this case, current issupplied for example by a storage battery or by a rectifier following the transformer, and the motor 66 in the locomotive is a direct current motor.

As is customary in electric toy railroads, the running rails are grounded, that is, the respective terminal of the secondary winding of the transformer 53 according to FIG. 4 is grounded.

In FIG. 6 is shown another application of the circuit principle according to FIG. 1. This is again the circuit diagram of an electric toy railroad, having two running rails 75 grounded and an insulated central conductor 76 to which voltage is applied. An electrically driven rail vehicle 77, for example a locomotive, has wheels '78 running on the running rails 75, while a current collector contact 79 slides on the third rail 76. The motor 88 of the vehicle is connected between a wheel 78 and the slip contact 79. The rails are fed in known manner, for example through a transformer (not shown) connected to the power supply.

The installation according to FIG. 6 further comprises a console 81 which is provided with control switch es 82 and 83 and in which the oscillator according to FIG. 1 is housed. From the oscillator of this console 81 are fed three acoustical signal generators 84A, 84B, 84C as well as a signal generator 85 arranged on the vehicle. These signal generators are shown as loudspeakers. The three signal generators 84A, 84B, 84C are stationary and are connected to the console 81 by a line 88a and by means of a contact 86 for each to the running rail 75, i.e-. to ground.

The signal generator 85 on the vehicle is con- 6 nected through a contact 87, which slides on an insulated conductor 88, to the console 81, While its other terminal is connected through wheel 78 with the running rail ground).

As shown in FIG. 6, the contacts 86 of the signal generator 84A, 84B and 84C are arranged in the immediate vicinity of one running rail 75, but separated therefrom, in such manner that on passing the rail sections A, B, C, the vehicle wheels establish an electrically conducting contact between the running rail and the stationary contacts 86. Therefore, if for example switch 82 of the console 81 is closed, the oscillator is energized and oscillates, but at first the audio frequency current is not transmitted to the signal generators. When the wheels of the rail vehicle pass over one of the rail sections A, B, C, the respective signal generator 84A, 84B, or 84C is set in operation by conductive connection of the contact 86 with the running rail 75. Two different rail vehicles may simultaneously energize two signal generators, for example if one vehicle passes rail section A and the other passes rail section B.

The other switch 83 of console 81 may be so arranged that with it one can switch on selectively signal generator 84A or 84B. The signal generator on the vehicle may be operated either by another switch (not shown), manually or automatically, in that in line 88 a switch is provided which is connected to a railroad signal set up on the track and is operated thereby. Naturally also other switching combinations for the signal generators are also possible.

The contact 87 on the rail vehicle 77 may be, for example, a current collector sliding over an overhead line 88, or a contact provided externally on the vehicle, which as a certain track point is passed, touches the terminal of line 88 installed there.

In FIG. 7 is shown another application of the circuit principle according to FIG. 2 to the electric installation of a toy railroad. The oscillator according to FIG. 2 is designated by the reference numeral 89. From one secondary winding of this oscillator, audio frequency current is supplied to the rail tracks 92 and to the third rail 94 and hence to the circuit of the vehicle motor through the schematically represented wheels and the current collecting bar 93 of the vehicle 91. The acoustical signal generator 96 is again connected in parallel with the vehicle motor 90. Power for the oscillator 89 and traction current for the rails is provided by a transformer 95. The audio frequency current superimposed on the traction current directly actuates the loudspeaker 96 and is prevented by the impedance of the vehicle motor 90 from passage through the motor circuit. A condenser 97 permits the feeding of the audio frequency oscillator current into the rails even when the traction current is cut off, that is, when the rail vehicle stands still.

A signal generator 96 on the vehicle can be operated in any desired manner and in any desired position of the vehicle on the rails, for example by a contact 98 to be operated by hand or automatically through a contact 99 arranged at a point of the track next to the running rail track 92 which upon passage of the wheels of the vehicle is telrgiporarily electrically connected with the running rail trac The oscillator 89 may be fed through an independent alternating or direct current source instead of through the transformer 95.

The tone and timbre of the acoustical signal generator can be modified at will in a simple manner by variation of resistance and/or the winding values of the transformer of the oscillator, to imitate for example the whistle of a locomotive or the horn of an automobile. The described electrical installation can, of course, be applied readily also to electrically operated toy cars and is distinguished by a very simple design. Moreover, the required electrical components are simple and cheap. With the circuit principle according to FIG. 2, moreover, an additional conductor of the audio frequency current is effected exclusively through the traction current circuit. By coupling the switch contacts for the signal generators with other devices of the railroad installation, numerous original effects can be obtained, for example so that the electric locomotive always whistles when it must stop in front of a closed block signal or before it passes a grade crossing.

In the case of independent toy vehicles not limited to tracks, the oscillator and the acoustical signal generator may be mounted jointly on the vehicle and be switched on either by hand or automatically.

In addition, the oscillator and acoustical signal generator may be installed as a combined small unit in other toys, such as dolls, animals, toy musical instruments, etc. In all cases, the tone of the signal generator can be adapted in a surprising manner to the nature of the toy. In the case of a toy musical instrument, for example, a cam disk may be provided with which the frequencydetermining elements of the oscillator circuit or of the signal generator are variable for the production of dilfen ent sounds, or several oscillators may be provided which can be actuated selectively for the production of a melody.

In the household and in the industry the acoustical signal generator according to the invention can be used in many ways as a bell, calling signal, warning signal, etc. and has the advantage that it can easily be installed everywhere as a small, independent and inexpensive unit.

What I claim is:

1. An acoustical generating circuit comprising: acoustical signal means;

voltage supply means having opposite terminals;

a transistor having a base electrode and two working electrodes comprising a collector and an emitter;

a transformer having a primary with at least two terminals and a secondary with two secondary Windings, and in which one of said secondary windings is connected between the base electrode and one working electrode of said transistor, and said signal means is connected with the other said secondary windings and having also at least two terminals;

a primary circuit comprising means connecting one terminal of said primary with one terminal of said voltage supply means, means connecting another terminal of said primary with one Working electrode of said transistor and means connecting the other working electrode of said transistor with the opposite terminal of said power supply means, and

a secondary circuit comprising means including an impedance connecting one terminal of said secondary with one working electrode of said transistor and connecting another terminal of said secondary with said base electrode of said transistor; said acoustical signal means being connected in said secondary circuit for operation by secondary current of said transformer.

2. An acoustical generating circuit according to claim 1, in which said signal means comprises a low impedance loud speaker capacitatively coupled with said other secondary winding.

3. An acoustical generating circuit according to claim 1, in which said transformer is over coupled.

4. An acoustical generating circuit according to claim 1, in which said voltage supply means comprises a power supply transformer having a primary and a secondary and means for rectifying alternating current supplied by the secondary of said power supply transformer.

5. An electrical circuit for systems having tracks. and rolling stock including at least one electrically propelled vehicle, comprising a first transformer having a primary and a secondary with at least two output terminals;

means for connecting said primary to a supply of alternating current;

means for connecting said secondary to said tracks for supplying power to said vehicle and including means for controlling the stopping, starting andspee d of said vehicle;

a transistor having a base electrode and two working electrodes comprising a collector and an emitter;

a second transformer having a primary with at least two terminals and a secondary with at least one secondary winding and at least two terminals;

a primary circuit comprising means connecting one terminal of said primary of said second transformer with one working electrode of said transistor, means connecting the other of said terminals of the primary of said second transformer with an output terminal of said first transformer and means connecting the other working electrode of said transistor with another output terminal of said first transformer, at

least one of said last two connecting means comprising rectifying means;

a secondary circuit comprising means including an impedance connecting one terminal of said secondary of said second transformer with one working electrode of said transistor and connecting another terminal of said last mentioned secondary with said base electrode of said transistor, and

a loud speaker connected in the secondary circuit of said second transformer for operation by secondary.

current from said second transformer.

6. A circuit according to claim 5 in which said speaker is stationary and further comprising contact means associated with said track and actuated by the passage of said rolling stock for controlling the energization of said speaker by said secondary current.

7. A circuit according to claim 5, in which said vehicle is propelled by an electric motor and in which said motor is electrically connected with said tracks through an inductance of a value substantially blocking the feeding to said motor of audio frequency current from said secondary circuit of said second transformer.

8. A circuit according to claim 5, in which said vehicle is propelled by an electric motor and in which said loud speaker is connected in parallel with said motor and has an impedance much smaller than the motor circuit.

9. An electrical circuit for a system having tracks and rolling stock including at least one vehicle having an electric motor for propulsion comprising means connected to said tracks for supplying to said tracks traction current for said motor;

an audio frequency oscillator circuit connected to said tracks for supplying audio frequency current to said tracks superimposed on said traction current, said oscillator-circuit comprising;

a transistor having a base electrode and two working electrodes comprising a collector and an emitter;

a transformer having a primary with at least two terminals and a secondary with at least one secondary winding and at least two terminals,

a primary circuit comprising means connecting one terminal of said primary with one working electrode of said transistor;

means connecting the other of said terminals of said primary with a direct current source,

means connecting the other of said working electrodes of said transistor with said direct current source,

a secondary circuit comprising means including an impedance connecting one terminal of said secondary with one working electrode of said transistor and connecting another terminal of said secondary with said base electrode of said transistor;

means connecting said secondary circuit with said track;

acoustical signal means;

means for electrically connecting said motor to said track to receive traction current therefrom, and;

means for electrically connecting said signal means to said track in parallel with said motor to receive from 9 said track audio frequency current supplied by said 2,882,834 oscillator circuit. 3,061,973 3,168,730 References Cited 3,277,465 UNITED STATES PATENTS 5 3,278,861 2,292,565 8/1942 Jordan 104-150 2,708,885 5/1955 Smith et a1. 104150 2,791,972 5/1957 Smith 46-232 X 2,826,996 3/1958 Smith 46-232 Smith 46-232 Oberdorf 46-232 Baldinelli 340-392 Potter 340-384 Cunningham 331-112 NEIL C. READ, Primary Examiner. THOMAS A. ROBINSON, Examiner. I. LEVIN, Assistant Examiner. 

1. AN ACOUSTICAL GENERATING CIRCUIT COMPRISING: ACOUSTICAL SIGNAL MEANS; VOLTAGE SUPPLY MEANS HAVING OPPOSITE TERMINALS; A TRANSISTOR HAVING A BASE ELECTRODE AND TWO WORKING ELECTRODES COMPRISING A COLLECTOR AND AN EMITTER; A TRANSFORMER HAVING A PRIMARY WITH AT LEAST TWO TERMINALS AND A SECONDARY WITH TWO SECONDARY WINDINGS, AND IN WHICH ONE OF SAID SECONDARY WINDINGS IS CONNECTED BETWEEN THE BASE ELECTRODE AND ONE WORKING ELECTRODE OF SAID TRANSISTOR, AND SAID SIGNAL MEANS IS CONNECTED WITH THE OTHER SAID SECONDARY WINDINGS AND HAVING ALSO AT LEAST TWO TERMINALS; A PRIMARY CIRCUIT COMPRISING MEANS CONNECTING ONE TERMINAL OF SAID PRIMARY WITH ONE TERMINAL OF SAID VOLTAGE SUPPLY MEANS, MEANS CONNECTING ANOTHER TERMINAL OF SAID PRIMARY WITH ONE WORKING ELECTRODE OF SAID TRANSISTOR AND MEANS CONNECTING THE OTHER WORKING ELECTRODE OF SAID TRANSISTOR WITH THE OPPOSITE TERMINAL OF SAID POWER SUPPLY MEANS, AND A SECONDARY CIRCUIT COMPRISING MEANS INCLUDING AN IMPEDANCE CONNECTING ONE TERMINAL OF SAID SECONDARY WITH ONE WORKING ELECTRODE OF SAID TRANSISTOR AND CONNECTING ANOTHER TERMINAL OF SAID SECONDARY WITH SAID BASE ELECTRODE OF SAID TRANSISTOR; SAID ACOUSTICAL SIGNAL MEANS BEING CONNECTED IN SAID SECONDARY CIRCUIT FOR OPERATION BY SECONDARY CURRENT OF SAID TRANSFORMER. 